1. Field of the Invention
The present invention relates to a color spatial light modulator and a color printer using the same. More particularly, the invention relates to a color spatial light modulator having small size mirrors disposed in a line or matrix, for each which the light reflection direction is variable for projecting a specific color spot light, and to a color printer using such a color spatial light modulator for image formation.
2. Description of the Related Art
A spatial light modulator has a function of deflecting a propagation direction of incident light, and so it is used, for example, as an on/off controller of a laser optical system for controlling propagation of a laser beam. Conventionally, an ultrasonic light modulator has been used which deflects a laser beam by ultrasonic wave. Recently, a mirror type spatial light modulator has been proposed which has a number of small size mirrors (hereinafter called micromirrors), the tilt angle of each micromirror being changed to control deflection. Mirror type spatial light modulators include digital micromirror devices (DMD) which tilt each micromirror by electrostatic force, piezoelectric type drive micromirror devices (AMA) which tilt each micromirror by mechanical deformation of a fine piezoelectric element, and the like.
For example, a digital micromirror device has a static RAM (SRAM) on each memory cell of which a micromirror capable of swinging is formed by semiconductor integration techniques. Mirror drive data of one bit, when written to each memory cell, tilts the micromirror in a positive direction or in a negative direction to change the light reflection direction. The principle and applications of such a digital micromirror device are described in a monthly magazine "O plus E", October, 1994, pp. 90-94.
As one of the applications, this document describes a sequential field type color video projector. In this color video projector, illumination light from a white light source passes through a color filter disk and enters a digital micromirror device. The color filter disk has three sectors of red, green, and blue filters. The digital micromirror device has a number of micromirrors disposed in matrix. As "1" is written to a memory cell, the micromirror tilts by an angle +.theta. from a horizontal plane, and as "0" is written, it tilts by an angle -.theta..
If the red filter is set in front of the white light source, red image data of one frame is written to the digital micromirror device. As the red image data of "1" is stored in a memory cell, this micromirror reflects the red light transmitted through the red filter toward a projection lens, whereas as the red image data of "0" is stored, the micromirror tilts by the angle -.theta. and the red light is reflected toward a light absorption plate.
The digital micromirror device generates a number of red spot lights depending on the tilt angle of each micromirror disposed in matrix. One red light spot corresponds to one red pixel. A red image of one frame constituted of these red light spots is projected via a projector lens onto a screen. The grey scale of a red image changes with a time period during which the tilt angle takes +.theta..
Next, green image data is written to the digital micromirror device, and thereafter the green filter is set in front of the white light source. A green image of one frame generated by the digital micromirror device is projected onto the screen. Thereafter, blue image data is written and a blue image of one frame is projected from the digital micromirror device onto the screen. While the color filter disk is rotated at high speed, image data of each color is written to the digital micromirror device synchronously with the timing of the setting of each color filter so that three color images are sequentially projected onto the screen at high speed and an image of full color synthesized on the screen can be observed.
A conventional mirror type spatial light modulator requires a rotatable color filter disk in order to display or record a color image. This color filter disk complicates the structure of a video projector and makes the projector bulky.
Known color printers for recording a color image on a photosensitive material include a CRT type, a laser type, a liquid crystal type, and the like. The CRT type color printer requires a large CRT and a complicated CRT driver circuit. The laser type color printer basically performs line exposure so that the intensity of a laser beam is modulated in the unit of pixel. This takes a long time for intensity modulation. Since an exposure time of one pixel is short, a reciprocity law failure of photosensitive material may occur. Compensation for this is very cumbersome. The liquid crystal type color printer requires a light source of high intensity because the transmittance of each pixel is small. Furthermore, since an aperture efficiency (vignetting factor) of each pixel is small, the image quality is not good.
The micromirror device has advantages of a low light attenuation coefficient and a large aperture efficiency. It is therefore advantageous if the micromirror device is utilized in a color printer. However this color printer using the micromirror device uses a rotatable color filter disk and the structure is made complicated.